1. Field of the Invention
The present invention relates to both a method and an apparatus for controlling the signal/noise ratio of optical add/drop signals of an optical WDM transmission signal in an optical WDM transmission system having a transmitter unit, a receiver unit and at least one optical add/drop multiplexer for transmitting optical express signals and at least one optical add/drop signal. Here, the signal/noise ratios of the express signals are determined in the receiver unit of the WDM transmission system, and the signal levels of the express signals are controlled in the transmitter unit as a function of the signal/noise ratios determined.
2. Description of the Prior Art
In existing and future optical transmission systems, in particular WDM (WDM=Wavelength Division Multiplexing) transmission systems, optical data transmission signals with different wavelengths are used for setting up data connections. At the request of network customers, network operators will make available, for example for a short time, optical logic connections between any desired connecting points within the optical transmission system; i.e., some of the existing optical connections in an optical transmission system have a static character and others have only short service lives.
Cross connectors or through-connecting devices, which have operated electronically, are known for creating such short-term optical connections. Automatically configurable optical through-connecting devices (also referred to as optical add/drop multiplexers (OADM) will, in particular, be provided in the future in the respective concentration points of the communications network in order to utilize the existing network capacities. Using such optical add/drop multiplexers (OADM), one or more WDM channels or WDM signals are removed or dropped from the optical composite signal or WDM transmission signal in the respective concentration points of the communications network, and others are added. Such WDM signals are referred to as local add/drop signals, and WDM signals, which are transmitted from a transmitter unit to a receiver unit over the entire optical transmission path, i.e. at least one optical add/drop multiplexer, are known under the designation express signals.
In order to optimize the optical signal/noise ratio of an optical n×10 Gbit/s transmission path, and thus the optical performance of individual WDM signals, in particular express signals of this kind, the express signals are pre-emphasized at the input of the optical WDM transmission path, i.e. in the transmitter unit, as a function of the properties of the optical WDM transmission path, in particular of the nonlinear properties, and of the optical repeaters, in terms of their signal power with attenuators in such a way that the optical signal/noise (OSNR) ratio has an optimized value at the end, i.e. in the receiver unit, of the optical WDM transmission path. An optimized signal/noise ratio for all the optical WDM signals, in particular the express signals, has the same value or the same amplitude for each WDM signal. The transmitter variance, for example that of the optical transmission signal, are compensated within the optical transmission system by such compensation of the different optical path properties on an individual basis for the respective WDM signal. For this purpose, methods for controlling the signal/noise ratio of express signals are known, in particular from the publication by A. R. Chraplyvy, et. al. “End to end equalization experiments in amplified WDM light wave systems”, IEEE Photonics Technology Letters, Vol. 4, No. 4, April 1993 and from “Unequalization in amplified WDM light wave transmissions systems”, IEEE Photonics Technology Letters, Vol. 4, No. 8, August 1992, and from the European Patent Application 92310342.8.
In such methods, the signal/noise ratio is controlled, in each case, by the attenuation of the signal power of the express signals at the start of the optical WDM transmission path; i.e., in the transmitter unit. However, such methods are not suitable for the automatic signal/noise ratio control of local add/drop signals, which are added to the optical composite signal in, for example, an optical add/drop multiplexer; i.e., the automatic control as in the case of the express signals cannot be carried out in accordance with the known methods.
For this reason, in previous transmission systems with transmission rates of 2.5 Gbit/s and add/drop functionality, local add/drop signals are injected at the injection point, i.e. in the transmitter unit or in the add/drop multiplexer, with a permanently predefined signal level. This permanently predefined signal level is selected such that the nonlinear effects occurring during the transmission of optical signals, for example the stimulated Raman scattering or the cross-phase modulation, in the optical fiber are kept low. Furthermore, all the incoming WDM signals are amplified in accordance with the received signal power via the optical fiber repeaters provided in the optical transmission system, but the signal noise added by the amplification is independent of the received signal strength here, which leads to a degradation of the signal/noise ratio OSNR for WDM signals having a low signal power. However, in optical transmission systems with data transmission rates higher than 10 Gbit/s, the optical signal/noise ratio is decisive, in particular for the transmission path length which can be spanned without regeneration, so that to operate such high-bit-rate optical transmission systems it is necessary to optimize the optical signal/noise ratio of the locally injected add/drop signals.
An object to which the present invention is directed, therefore, is to optimize the optical signal/noise ratio of local add/drop signals or disclosing a control method with which the optimum optical signal/noise ratio of local add/drop signals can be set.